1994 Chevy G20 Conv. Van will not warm up

I have a 1994 Chevy G20 conversion van converted by Tiara. I have owned it for a year. The engine temperature never seems to warm up. In the summer the temperature gets to about 160. Now it's winter and it takes forever for the needle to move just a little bit. The inside of the van heats up after 20 minutes and pumps out hot air.

I have replaced the thermostat twice. I made sure to put the spring side towards the engine. I replaced the temperature sensor. I have flushed the system twice. Anybody have any ideas??? Everyone I talk to just scratch their head and say they have no idea.